NFL rule book:
ARTICLE 1. COACHES' CHALLENGE
In each game, a team will be permitted two challenges that will initiate Instant Replay reviews. The Head Coach will initiate a challenge by throwing a red flag onto the field of play before the next legal snap or kick. Each challenge will require the use of a team timeout. If a challenge is upheld, the timeout will be restored. A challenge will only be restored if a team is successful on both of its challenges, in which case it shall be awarded a third challenge, but a fourth challenge will not be permitted under any circumstances.
Except for Pete the Cheat when playing at home.